heater matrix on a renault megane-1997 model?

Where can i locate the heater matrix on a 97 renault megane 1.6. I have had a leaking radiator for 12 months,filled it with radweld and after a month or so fan started blowing cold. Now fitted new rad but fan still cold so wondered if matrix is bunged up with radweld. Need to locate matrix to try and flush it out!